1. Field of the Invention
The present invention relates to an in-circuit emulator, and more particularly to a CPU identifier circuit of such an in-circuit emulator adapted to select operation of a control circuit for different types of microprocessors (hereinafter, referred to as a CPU) different in portions of constituent elements thereof from each other.
2. Description of the Prior Art
Some CPUs have substantially the same package signal and pin arrangement.
CPUs each having a data bus of 16 bits of 8 bits are typically employed, which CPUs resemble each other from the viewpoint of their appearances and electric properties with only slight differences among their control signals associated therewith.
FIG. 1 here shows a table which lists different portions among those similar CPUs when they are divided into two groups: CPU 1 and CPU 2.
pins 10 to 17 of the CPU 1 correspond to AD 15 to AD 8 in the table.
AD 8 to AD 15 serve as an address bus and a data bus by time sharing because of the data bus being contrived to be 16 bits, and furthermore serve for input/output operation when the data bus is operated.
The data bus is controlled by making AO "0" when the lower-rank data bus (D 0 to D 7) is employed, while it controlled by making HBE of a 51 pin "0" when the higher-rank data bus (AD 8 to AD 15) is employed.
Since the data bus of the CPU 2 is adapted to have its width of 8 bits, those pins of from 10 to 17 serve only as outputs of the address bus (A 15 to A 8).
In addition, the CPU 2 is not in need of controlling higher- and lower-rank data buses, and hence the pin 51 is fixed to "1".
CPUs 1 and 2 respectively include 68 pins among which nine pins are different, as shown in FIG. 1.
Accordingly, in-circuit emulators corresponding to those CPUs 1 and 2 can be united into one emulator with ease.
The in-circuit emulator here means one connected with an actual apparatus, instead of a CPU of the actual apparatus, for checking a memory and I/O ports of the actual apparatus and for debugging a program thereof.
Since a signle in-circuit emulator can correspond to a plurality of CPUs as described above, the emulator can conveniently be operated, and the development and fabrication costs of the in-circuit emulator per CPU can be reduced.
Such an in-circuit emulator can be realized by using as a CPU involved in the in-circuit emulator an object CPU.
Thereupon, operation of a peripheral control circuit of the CPU must be switched too, depending on the object CPU.
A block diagram of a portion of such a prior in-circuit emulator related to the present invention is illustrated in FIG. 2.
In the same figure, designated at 1 is a CPU, 3 is a control circuit, 4 is an actual apparatus, and 5 is external setting means.
The concerning in-circuit emulator includes the CPU 1, control circuit 3, and external setting means 5 shown in FIG. 2.
For the CPU 1, a CPU of the same type as that involved in the actual apparatus 4 is employed.
Conventionally, a manual switch and a software for control, etc., were employed as the external setting means 5 to switch the operation modes of the control circuit 3. However, the prior external setting means 5 is not only complicated in its operation, but erroneous operation of the external setting means 5 may sometimes cause, although the CPU 1 of FIG. 1 for example is to be employed as the emulator CPU, this emulator CPU to be set as the CPU 2 of FIG. 1. There is produced as a result a problem that this situation prevents the in-circuit emulator from operating correctly and produces damages to circuit elements.